Method for forming carbon nanotube film

ABSTRACT

A method for forming a carbon nanotube film is provided. An elastic substitute substrate and a carbon nanotube array transferred on a surface of the elastic substitute substrate are used. The carbon nanotube array is configured for drawing a carbon nanotube film therefrom. The carbon nanotube film has carbon nanotubes joined end to end. The elastic substitute substrate is stretched along a plurality of directions to increase lengths of the carbon nanotube array along the plurality of directions. The carbon nanotube film is drawn from the stretching carbon nanotube array.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims all benefits accruing under 35 U.S.C. §119 fromChina Patent Application No. 201410159460.9, filed on Apr. 14, 2014 inthe China Intellectual Property Office, the contents of which are herebyincorporated by reference. This application is related to applicationsentitled, “METHOD FOR FORMING CARBON NANOTUBE FILM”, filed ______ (Atty.Docket No. US56119) and “METHOD FOR FORMING CARBON NANOTUBE FILM”, filed______ (Atty. Docket No. US56120).

FIELD

The subject matter herein generally relates to methods for formingcarbon nanotube films.

BACKGROUND

Carbon nanotube structures can be fabricated by drawing from a carbonnanotube array grown on a growing substrate (e.g., silicon wafer), asdisclosed by U.S. Pat. No. 8,048,256 to Feng et al. The carbon nanotubefilm is free standing and includes a plurality of carbon nanotubesjoined end-to-end by van der Waals attractive force therebetween. Thecarbon nanotubes in the carbon nanotube film are substantially alignedalong the lengthwise direction of the carbon nanotube film, and thus,the carbon nanotube film has good thermal and electrical conductivityalong the direction of the aligned carbon nanotubes. The carbon nanotubefilm is substantially transparent and can be used as a conductive thinfilm. Therefore, the carbon nanotube film can be used in many differentfields, such as touch panels, liquid crystal displays, speakers, heatingdevices, thin film transistors, cables, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

Implementations of the present technology will now be described, by wayof example only, with reference to the attached figures, wherein:

FIG. 1 is a schematic top view of an embodiment of a method for forminga carbon nanotube film.

FIG. 2 shows a scanning electron microscope (SEM) image of a carbonnanotube film drawn from a carbon nanotube array.

FIG. 3 shows carbon nanotubes joined end-to-end.

FIG. 4 is a schematic side view of an embodiment of a method fortransferring a carbon nanotube array.

FIG. 5 is a schematic side view of another embodiment of the method fortransferring the carbon nanotube array.

FIG. 6 is a schematic side view of yet another embodiment of the methodfor transferring the carbon nanotube array.

FIG. 7 is a schematic side view of yet another embodiment of the methodfor transferring the carbon nanotube array.

FIG. 8 is a schematic side view of yet another embodiment of the methodfor transferring the carbon nanotube array.

FIG. 9 is a schematic side view of one embodiment of the carbon nanotubearray before and after stretching.

FIG. 10 is a schematic top view of one embodiment of grooves on thecarbon nanotube array.

FIG. 11 is a schematic top view of another embodiment of the grooves onthe carbon nanotube array.

FIG. 12 is a schematic side view of an embodiment of a method forforming grooves on the carbon nanotube array transferred on an elasticsubstitute substrate.

FIG. 13 is a schematic side view of an embodiment of the method forforming the carbon nanotube film.

DETAILED DESCRIPTION

The disclosure is illustrated by way of example and not by way oflimitation in the figures of the accompanying drawings in which likereferences indicate similar elements. It should be noted that referencesto “another,” “an,” or “one” embodiment in this disclosure are notnecessarily to the same embodiment, and such references mean “at leastone.”

It will be appreciated that for simplicity and clarity of illustration,where appropriate, reference numerals have been repeated among thedifferent figures to indicate corresponding or analogous elements. Inaddition, numerous specific details are set forth in order to provide athorough understanding of the embodiments described herein. However, itwill be understood by those of ordinary skill in the art that theembodiments described herein can be practiced without these specificdetails. In other instances, methods, procedures and components have notbeen described in detail so as not to obscure the related relevantfeature being described. Also, the description is not to be consideredas limiting the scope of the embodiments described herein. The drawingsare not necessarily to scale and the proportions of certain parts havebeen exaggerated to better illustrate details and features of thepresent disclosure.

Several definitions that apply throughout this disclosure will now bepresented.

The term “contact” is defined as a direct and physical contact. The term“substantially” is defined to be essentially conforming to theparticular dimension, shape, or other description that is described,such that the component need not be exactly conforming to thedescription. The term “comprising,” when utilized, means “including, butnot necessarily limited to”; it specifically indicates open-endedinclusion or membership in the so-described combination, group, series,and the like.

Referring to FIG. 1, the present disclosure is described in relation toa method for forming a carbon nanotube film.

In block 51, a carbon nanotube array 10 transferred on an elasticsubstitute substrate 30 is provided. The carbon nanotube array 10 is ina state that is capable of having the carbon nanotube film 40 drawntherefrom. The carbon nanotube film 40 can be a free-standing structureincluding a plurality of carbon nanotubes joined end-to-end by van derWaals attractive force therebetween.

In block S2, at least one groove 12 is formed on the carbon nanotubearray 10. Block S2 is optional.

In block S3, the elastic substitute substrate 30 is stretched along aplurality of directions to increase lengths of the carbon nanotube array10 along the plurality of directions.

In block S4, the carbon nanotube film 40 is drawn from the carbonnanotube array 10.

[Transferring of Carbon Nanotube Array]

One embodiment of a method for transferring a carbon nanotube array 10to the elastic substitute substrate 30 includes blocks S11 and S12.

In block S11, a growing substrate 20 having the carbon nanotube array 10grown thereon is provided.

In block S12, the carbon nanotube array 10 is transferred from thegrowing substrate 20 onto the elastic substitute substrate 30. The stateof the carbon nanotube array 10 before, during, and after the transferonto the elastic substitute substrate 30 is still capable of having thecarbon nanotube film 40 drawn therefrom.

The carbon nanotube array 10 is grown on the growing substrate 20 by achemical vapor deposition (CVD) method. The carbon nanotube array 10includes a plurality of carbon nanotubes oriented substantiallyperpendicular to a growing surface of the growing substrate 20. Thecarbon nanotubes in the carbon nanotube array 10 are closely bondedtogether side-by-side by van der Waals attractive forces. By controllinggrowing conditions, the carbon nanotube array 10 can be essentially freeof impurities such as carbonaceous or residual catalyst particles.Accordingly, the carbon nanotubes in the carbon nanotube array 10 areclosely contacting each other, and a relatively large van der Waalsattractive force exists between adjacent carbon nanotubes. The van derWaals attractive force is so large that when drawing a carbon nanotubesegment (e.g., a few carbon nanotubes arranged side-by-side), adjacentcarbon nanotube segments can be drawn out end-to-end from the carbonnanotube array 10 due to the van der Waals attractive forces between thecarbon nanotubes. The carbon nanotubes are continuously drawn to form afree-standing and macroscopic carbon nanotube film 40. The carbonnanotube array 10 that can have the carbon nanotube film 40 drawntherefrom can be a super aligned carbon nanotube array. A material ofthe growing substrate 20 can be P-type silicon, N-type silicon, or othermaterials that are suitable for growing the super aligned carbonnanotube array.

The carbon nanotube film 40 drawn from the carbon nanotube array 10includes a plurality of carbon nanotubes joined end-to-end and can be afree-standing carbon nanotube film. The carbon nanotube film includes aplurality of carbon nanotubes substantially aligned along the samedirection.

Referring to FIG. 2 and FIG. 3, the carbon nanotube film 40 can includeor consist of a plurality of carbon nanotubes. In the carbon nanotubefilm 40, the overall aligned direction of a majority of the carbonnanotubes is substantially aligned along the same direction parallel toa surface of the carbon nanotube film 40. A majority of the carbonnanotubes are substantially aligned along the same direction in thecarbon nanotube film 40. Along the aligned direction of the majority ofcarbon nanotubes, each carbon nanotube is joined to adjacent carbonnanotubes end to end by van der Waals attractive force therebetween,whereby the carbon nanotube film 40 is capable of being free-standingstructure. There may be a minority of carbon nanotubes in the carbonnanotube film 40 that are randomly aligned. However, the number of therandomly aligned carbon nanotubes is very small and does not affect theoverall oriented alignment of the majority of carbon nanotubes in thecarbon nanotube film 40. Some of the majority of the carbon nanotubes inthe carbon nanotube film 40 that are substantially aligned along thesame direction may not be exactly straight, and can be curved at acertain degree, or not exactly aligned along the overall aligneddirection by a certain degree. Therefore, partial contacts can existbetween the juxtaposed carbon nanotubes in the majority of the carbonnanotubes aligned along the same direction in the carbon nanotube film40. The carbon nanotube film 40 can include a plurality of successiveand oriented carbon nanotube segments. The plurality of carbon nanotubesegments are joined end to end by van der Waals attractive force. Eachcarbon nanotube segment includes a plurality of carbon nanotubessubstantially parallel to each other, and the plurality of paralleledcarbon nanotubes are in contact with each other and combined by van derWaals attractive force therebetween. The carbon nanotube segment has adesired length, thickness, uniformity, and shape. There can beclearances between adjacent and juxtaposed carbon nanotubes in thecarbon nanotube film 40. A thickness of the carbon nanotube film 40 atthe thickest location is about 0.5 nanometers to about 100 microns(e.g., in a range from 0.5 nanometers to about 10 microns).

The term “free-standing” includes, but is not limited to, a carbonnanotube film 40 that does not need to be supported by a substrate. Forexample, a free-standing carbon nanotube film 40 can sustain the weightof itself when it is hoisted by a portion thereof without anysignificant damage to its structural integrity. If the free-standingcarbon nanotube film 40 is placed between two separate supporters, aportion of the free-standing carbon nanotube film 40 suspended betweenthe two supporters can maintain structural integrity. The free-standingcarbon nanotube film 40 is realized by the successive carbon nanotubesjoined end to end by van der Waals attractive force.

In the present disclosure, the growing of the carbon nanotube array 10and the drawing of the carbon nanotube film 40 are processed ondifferent structures (i.e., the growing substrate 20 and the elasticsubstitute substrate 30). The elastic substitute substrate 30 fordrawing the carbon nanotube film 40 can be made of low-price materials,and the growing substrate 20 can be recycled quickly. Thus, productionof the carbon nanotube film 40 can be optimized.

Referring to FIG. 4, the elastic substitute substrate 30 can be a soft,elastic, and solid substrate. The elastic substitute substrate 30 has asurface 302 to accept the carbon nanotube array 10 thereon. Duringtransferring of the carbon nanotube array 10 from the growing substrate20 to the surface 302 of the elastic substitute substrate 30, the stateof the carbon nanotube array 10 is still capable of drawing the carbonnanotube film 40 from the carbon nanotube array 10 on the elasticsubstitute substrate 30. That is, the carbon nanotube array 10transferred to the elastic substitute substrate 30 is still a superaligned carbon nanotube array.

The carbon nanotube array 10 is arranged upside down on the surface 302of the elastic substitute substrate 30. The carbon nanotubes are grownfrom the growing surface 202 of the growing substrate 20 to form thecarbon nanotube array 10. The carbon nanotube includes a bottom endadjacent to or contacting the surface 202 of the growing substrate 20and a top end away from the surface 202 of the growing substrate 20. Thebottom ends of the carbon nanotubes form the bottom surface 102 of thecarbon nanotube array 10, and the top ends of the carbon nanotubes formthe top surface 104 of the carbon nanotube array 10. After the carbonnanotube array 10 is transferred to the elastic substitute substrate 30,the top surface 104 of the carbon nanotube array 10 is now adjacent toor contacting the surface 302 of the elastic substitute substrate 30,and the bottom surface 102 of the carbon nanotube array 10 is now awayfrom the surface 302 of the elastic substitute substrate 30.

In one embodiment, block S12 includes blocks A121 and A122.

In block A121, the elastic substitute substrate 30 and the carbonnanotube array 10 on the growing substrate 20 are brought together suchthat the surface 302 of the elastic substitute substrate 30 and the topsurface 104 of the carbon nanotube array 10 are contacting each other.

In block A122, the elastic substitute substrate 30 and the growingsubstrate 20 are moved away from each other, thereby separating thecarbon nanotube array 10 from the growing substrate 20.

The carbon nanotube array 10 can be transferred from the growingsubstrate 20 to the elastic substitute substrate 30 at room temperature(e.g., 10° C. to 40° C.).

The surface of the elastic substitute substrate 30 and the top surface104 of the carbon nanotube array 10 can be bonded by van der Waalsattractive forces, and a bonding force (F_(BC)) between the carbonnanotube array 10 and the elastic substitute substrate 30 is smallerthan the van der Waals attractive forces (F_(CC)) between the carbonnanotubes in the carbon nanotube array 10. Meanwhile, the bonding forceF_(BC) is larger than the bonding force (F_(AC)) between the carbonnanotube array 10 and the growing substrate 20, to separate the carbonnanotube array 10 from the growing substrate 20. Therefore,F_(AC)<F_(BC)<F_(CC) must be satisfied.

To satisfy F_(AC)<F_(BC)<F_(CC), the elastic substitute substrate 30 canhave a suitable surface energy and a suitable interface energy can existbetween the elastic substitute substrate 30 and the carbon nanotubearray 10. Thus, the elastic substitute substrate 30 can generate enoughbonding force (e.g., van der Waals attractive force) with the carbonnanotube array 10 simply by contacting the carbon nanotube array 10. Asuitable material of the elastic substitute substrate 30 must have asufficient bonding force F_(BC) (e.g., van der Waals attractive force)with the top surface 104 of the carbon nanotube array 10 to overcome thebonding force F_(AC) between the carbon nanotube array 10 from thegrowing substrate 20. The surface 302 of the elastic substitutesubstrate 30 can be substantially flat. In one embodiment, the materialof the elastic substitute substrate 30 is poly(dimethylsiloxane) (PDMS).

The elastic substitute substrate 30 can adhere to the carbon nanotubearray 10 without an adhesive binder and only by van der Waals attractiveforces. Although the adhesive binder can have a bonding force with thecarbon nanotube array greater than the bonding force between the carbonnanotube array 10 and the growing substrate 20, because the van derWaals attractive force between the carbon nanotubes in the carbonnanotube array 10 is small, the adhesive binder must have a bondingforce with the carbon nanotube array 10 sufficiently less than thebonding force F_(CC) between the carbon nanotubes in the carbon nanotubearray 10. Otherwise, the carbon nanotube film 40 cannot be drawn fromthe transferred carbon nanotube array 10. In blocks A121 and A122, theelastic substitute substrate 30 can always be in a solid state.

Referring to FIG. 5, in one embodiment, to satisfy F_(AC)<F_(BC)<F_(CC),the substitute substrate 30 can increase the surface area of the surface302 by using the microstructures 304, thus increasing the F_(BC). Theelastic substitute substrate 30 can have the surface 302 with aplurality of microstructures 304 located thereon. The microstructure 304can have a point shape and/or a long and narrow shape, and can beprotrusions and/or recesses. The cross section of the microstructures304 can be semicircular, rectangular, conical, and/or stepped. Themicrostructures 304 can be hemi-spheres, convex or concave columns,pyramids, pyramids without tips, and any combination thereof. In oneembodiment, the microstructures 304 can be parallel and spaced grooves.In another embodiment, the microstructures 304 can be uniformly spacedhemispherical protrusions. The plurality of microstructures 304 areuniformly distributed on the surface 304 of the elastic substitutesubstrate 30. In one embodiment, the surface 302 having themicrostructures 304 located thereon has a surface area of 30% to 120%more than a smooth surface. The surface 302 sufficiently contacts thetop surface 104 of the carbon nanotube array 10. The material of theelastic substitute substrate 30 is not limited and can be at least oneof plastic and resin, such as polymethyl methacrylate and/orpolyethylene terephthalate.

The height of the protrusion and the depth of the recess of themicrostructures 304 can be 0.5% to 10% of the height of the carbonnanotube array 10. In one embodiment, the height of the protrusion andthe depth of the recess can be in a range from about 5 microns to about50 microns. The surface 302 needs an overall flatness to sufficientlycontact the top surface 104 of the carbon nanotube array 10. Themicrostructures 304 can be formed on the surface 302 by laser etching,chemical etching, or lithography.

The microstructures 304 make the surface 302 of the elastic substitutesubstrate 30 rough. In block A121, when the recessed portion of thesurface 302 is in contact with the top surface 104 of the carbonnanotube array 10, the protruded portion of the surface 302 may slightlycurve the carbon nanotubes contacting the protruded portion. However,the microstructures 304 are small, so the curve is small, and when theelastic substitute substrate 30 and the growing substrate 20 areseparated, the carbon nanotubes can elastically restore to asubstantially straight shape and the carbon nanotube array 10 canrestore to its original height. Thus, the state of the carbon nanotubearray 10 is still capable of having the carbon nanotube structure 40drawn from the carbon nanotube array 10.

In block A121, to ensure almost all the top ends of the carbon nanotubesin the carbon nanotube array 10 have sufficient contact with the surfaceof the elastic substitute substrate 30, the elastic substitute substrate30 and the growing substrate 20 can be brought close enough. A distancefrom the surface 302 of the elastic substitute substrate 30 to thesurface 202 of the growing substrate 20 can be less than or equal to theheight of the carbon nanotube array 10 to apply a pressing force (f) tothe carbon nanotube array 10. The pressing force f cannot be too largeto ensure the state of the carbon nanotube array 10 is still capable ofdrawing the carbon nanotube film 40 when transferred to the elasticsubstitute substrate 30. The pressing force is not to press the carbonnanotubes down or vary the length direction of the carbon nanotubes inthe carbon nanotube array 10, otherwise the state of the carbon nanotubearray 10 could change. Thus, the distance between the surface 302 of theelastic substitute substrate 30 and the surface 202 of the growingsubstrate 20 cannot be too small and should be larger than an extremevalue. The extreme value is a value that causes the state of the carbonnanotube array 10 to be unable to draw the carbon nanotube film 40.

However, the pressing force is difficult to control, and the height ofthe carbon nanotube array 10 is often in tens of microns to hundreds ofmicrons. If the pressing force is too large, the carbon nanotubes in thearray 10 may be pressed down. Referring to FIG. 6, in one embodiment, aspacing element 22 is provided. The elastic substitute substrate 30 isspaced from the growing substrate 20 by the spacing element 22. Thespacing element 22 is used to limit the distance between the surface 302of the elastic substitute substrate 30 and the surface 202 of thegrowing substrate 20. The height of the spacing element 22 locatedbetween the elastic substitute substrate 30 and the growing substrate 20is smaller than or equal to the height of the carbon nanotube array 10and larger than the extreme value. A height distance (z) between thespacing element 22 and the carbon nanotube array 10 can exist. Thespacing element 22 is a solid member. In one embodiment, the spacingelement 22 is rigid. By controlling the height of the spacing element22, the distance between the elastic substitute substrate 30 and thegrowing substrate 20 can be precisely controlled. The height (m) of thespacing element 22 can be 0.9 times to 1 time of the height (n) of thecarbon nanotube array 10 (i.e., m=0.9n to n).

During the pressing of the carbon nanotube array 10, the carbonnanotubes in the carbon nanotube array 10 are still substantiallyperpendicular to the growing surface of the growing substrate 20. Whenthe height (m) is smaller than the height (n), the carbon nanotubes inthe carbon nanotube array 10 can be pressed to be curved slightly.However, the curve is small and when the elastic substitute substrate 30and the growing substrate 20 are separated, the carbon nanotubes canrestore the straight shape and the carbon nanotube array 10 can restorethe original height. Thus, the state of the carbon nanotube array 10 isstill kept to be capable of having the carbon nanotube film 40 drawnfrom the carbon nanotube array 10.

In one embodiment, the spacing element 22 is arranged on the growingsubstrate 20. In another embodiment, the spacing element 22 is arrangedon the elastic substitute substrate 30. In yet another embodiment, thespacing element 22 can be a part of the growing substrate 20 or theelastic substitute substrate 30. A shape of the spacing element 22 isnot limited and can be a block, a piece, a column, or a ball. There canbe a plurality of spacing elements 22 uniformly arranged around thecarbon nanotube array 10. The spacing element 22 can be a round circlearound the carbon nanotube array 10. In another embodiment, the spacingelements 22 are a plurality of round columns uniformly arranged aroundthe carbon nanotube array 10. The spacing element 22 can be used with orwithout the microstructures 304.

In block A122, a majority of the carbon nanotubes in the carbon nanotubearray 10 can be detached from the growing substrate 20 at the same timeby cutting means, or moving either the elastic substitute substrate 30or the growing substrate 20, or both, away from each other along adirection substantially perpendicular to the growing surface of thegrowing substrate 20. The carbon nanotubes of the carbon nanotube array10 are detached from the growing substrate 20 along the growingdirection of the carbon nanotubes. When both the elastic substitutesubstrate 30 and the growing substrate 20 separate, the two substratesboth moves along the direction perpendicular to the growing surface ofthe growing substrate 20 and depart from each other.

Referring to FIG. 7, in another one embodiment, block S12 includesblocks B121 to B124.

In block B121, the elastic substitute substrate 30 is placed on the topsurface 104 of the carbon nanotube array 10, and liquid medium 60 issandwiched between the surface 302 of the elastic substitute substrate30 and the top surface 104 of the carbon nanotube array 10.

In block B122, the liquid medium 60 between the elastic substitutesubstrate 30 and the carbon nanotube array 10 is solidified into a solidmedium 60′.

In block B123, the elastic substitute substrate 30 and the growingsubstrate 20 are moved away from each other, thereby separating thecarbon nanotube array 10 from the growing substrate 20.

In block B124, the solid medium 60′ between the elastic substitutesubstrate 30 and the carbon nanotube array 10 is removed by heating. Thecarbon nanotube array 10 is transferred from the growing substrate 20onto the elastic substitute substrate 30. The state of the carbonnanotube array 10, before, during, and after the transfer onto theelastic substitute substrate 30, and after the removal of the solidmedium 60′, is still capable of having the carbon nanotube film 40 drawntherefrom.

In block B121, the liquid medium 60 can be in a shape of fine droplets,mist, or film. The liquid medium 60 can spread on the entire top surface104. The liquid medium 60 can be water and/or organic solvents withsmall molecular weights that are volatile at room temperature or easilyevaporated by heating. The organic solvent can be selected from ethanol,methanol, and acetone. The liquid medium 60 has a poor wettability forcarbon nanotubes. Thus, when a small amount of the liquid medium 60 ison the top surface 104 of the carbon nanotube array 10, it cannotinfiltrate inside the carbon nanotube array 10 and will not affect thestate of the carbon nanotube array 10. A diameter of the liquid dropletand a thickness of the liquid film can be in a range from about 10nanometers to about 300 microns. The elastic substitute substrate 30 andthe top surface 104 of the carbon nanotube array 10 are both in contactwith the liquid medium 60.

In block B121, to maintain the state of the carbon nanotube array 10 ofbeing capable of drawing the carbon nanotube structure 40, the elasticsubstitute substrate 30 may apply a pressing force as small as possibleto the carbon nanotube array 10. The pressing force can satisfy0<f<2N/cm². The pressing force does not press the carbon nanotubes downor vary the length direction of the carbon nanotubes in the carbonnanotube array 10. The carbon nanotubes in the carbon nanotube array 10between the elastic substitute substrate 30 and the growing substrate 20are always substantially perpendicular to the growing surface of thegrowing substrate 20.

In one embodiment of the block B121, the liquid medium 60 is formed onthe top surface 104 of the carbon nanotube array 10. The liquid medium60 can be formed into fine droplets or a mist in the air and drop orcollect onto the top surface 104 of the carbon nanotube array 10. Theelastic substitute substrate 30 and the carbon nanotube array 10 on thegrowing substrate 20 are brought together such that the surface of theelastic substitute substrate 30 and the liquid medium 60 on the topsurface 104 are contacting each other.

In another embodiment of block B121, the liquid medium 60 is formed onthe surface of the elastic substitute substrate 30. The liquid medium 60can be formed into fine droplets or a mist in the air and drop orcollect onto the surface of the elastic substitute substrate 30. Theelastic substitute substrate 30 and the carbon nanotube array 10 on thegrowing substrate 20 are brought together such that the top surface 104of the carbon nanotube array 10 and the liquid medium 60 on the surfaceof the elastic substitute substrate 30 are contacting each other.

In block B122, the temperature of the liquid medium 60 can be decreasedto below the freezing point of the liquid medium 60. After the liquidmedium 60 is solidified, the elastic substitute substrate 30 and thecarbon nanotube array 10 can be firmly bonded together by the solidmedium 60′ therebetween. In one embodiment, water is frozen into icebelow 0° C.

In one embodiment, the lamination of the growing substrate 20, thecarbon nanotube array 10, the liquid medium 60, and the elasticsubstitute substrate 30 can be put into a freezer 70 with a temperaturebelow the freezing point to freeze the liquid medium 60.

Referring to FIG. 8, in another embodiment, when the liquid medium 60 isformed on the top surface 104 of the carbon nanotube array 10, atemperature of the elastic substitute substrate 30 can be decreased tobelow the freezing point before contacting the elastic substitutesubstrate 30 with the liquid medium 60. For example, the elasticsubstitute substrate 30 can be kept in the freezer 70 for a period oftime until the elastic substitute substrate 30 reaches a temperaturebelow the freezing point. Thus, when the elastic substitute substrate 30contacts the liquid medium 60 on the top surface 104 of the carbonnanotube array 10, the liquid medium 60 can be directly frozen intosolid medium 60′.

In block B123, due to the bonding between the carbon nanotube array 10and the elastic substitute substrate 30 by the solid medium 60′, theseparating of the two substrates can separate the carbon nanotube array10 from the growing substrate 20. During the separating, a majority ofthe carbon nanotubes in the carbon nanotube array 10 can be detachedfrom the growing substrate 20 at the same time by cutting means, ormoving either the elastic substitute substrate 30 or the growingsubstrate 20, or both, away from each other along a directionsubstantially perpendicular to the growing surface of the growingsubstrate 20. The carbon nanotubes of the carbon nanotube array 10 aredetached from the growing substrate 20 along the growing direction ofthe carbon nanotubes. When both the elastic substitute substrate 30 andthe growing substrate 20 separate, the two substrates both moves alongthe direction perpendicular to the growing surface of the growingsubstrate 20 and depart from each other.

In block B124, the heating can melt the solid medium 60′ into liquidmedium and dry the liquid medium between the elastic substitutesubstrate 30 and the carbon nanotube array 10. Or, the heating candirectly sublimate the solid medium 60′. The removal of the solid medium60′ does not affect the state of the carbon nanotube array 10. Due tothe thickness of the solid medium 60′ being small, after the removal ofthe solid medium 60′, the top surface 104 of the carbon nanotube array10 can be in contact with the surface of the elastic substitutesubstrate 30 and bonded by van der Waals attractive forces.

For drawing the carbon nanotube structure 40, the bonding force betweenthe carbon nanotube array 10 and the elastic substitute substrate 30should be small. In blocks B122 to B124, the bonding force is increasedby the solid medium 60′ to separate the carbon nanotube array 10 fromthe growing substrate 20 and decreased by removing the solid medium 60′before drawing the carbon nanotube structure 40. Thus, the material ofthe elastic substitute substrate 30 is not limited and can be at leastone of plastic and resin, such as polymethyl methacrylate and/orpolyethylene terephthalate.

[Stretching of Carbon Nanotube Array]

In block S3, the elastic substitute substrate 30 can be simultaneouslystretched along a plurality of directions substantially parallel to thesurface 302 of the elastic substitute substrate 30. Due to thestretching along the plurality of directions, the elastic substitutesubstrate 30 has deformations along the plurality of directions whichincrease the lengths of the elastic substitute substrate 30 along theplurality of directions. The carbon nanotube array 10 located on thesurface 302 of the elastic substitute substrate 30, and the carbonnanotube array 10 also has deformations along the plurality ofdirections which increase the lengths of the carbon nanotube array 10along the plurality of directions. The stretching along the plurality ofdirections can increase the lengths of the elastic substitute substrate30 along the plurality of directions in the same ratio, and increase thelengths of the carbon nanotube array 10 along the plurality ofdirections in the same ratio. The stretching along the plurality ofdirections can increase the size of the carbon nanotube array 10 (i.e.,increase the area of the bottom surface 102). In one embodiment, theelastic substitute substrate 30 can be simultaneously stretched alongtwo directions substantially perpendicular to each other. In anotherembodiment, the elastic substitute substrate 30 can be simultaneouslystretched along all the directions substantially parallel to the surface302 of the elastic substitute substrate 30. For example, the elasticsubstitute substrate 30 can cover a round supporter, such as a circularframe or a round plate. The round supporter has a size larger than thecarbon nanotube array 10 and smaller than the elastic substitutesubstrate 30. The carbon nanotube array 10 is coaxially arranged withthe round supporter. The edges of the elastic substitute substrate 30protrude out from the edge of the round supporter. The edges of theelastic substitute substrate 30 are pulled along a directionsubstantially perpendicular to the surface 302 of the elastic substitutesubstrate 30. Due to the support of the round supporter, the elasticsubstitute substrate 30 increases the lengths in all the directionssubstantially parallel to the surface 302.

Referring to FIG. 9, the carbon nanotube array 10 includes a pluralityof carbon nanotubes combined by van der Waals attractive forces. Due tothe stretching of the elastic substitute substrate 30, the distancesbetween bottom ends of the carbon nanotubes increase and some of thebottom ends may be spaced from each other. However, the carbon nanotubesare not completely straight, and can be curved. The adjacent carbonnanotubes can still have contacted portions of the sidewalls (as pointedby the arrows in FIG. 9). During the stretching, the contacted portionsof the carbon nanotubes decrease. However, the stretching degree of theelastic substitute substrate 30 can ensure that the carbon nanotubesstill have enough contacted portions that provide enough van der Waalsattractive forces to enable the carbon nanotube film 40 being drawn fromthe carbon nanotube array 10. The greater the lengths increased alongthe plurality directions of the carbon nanotube array 10, the smallerthe contact portions of the carbon nanotubes, and the smaller thedensity of the stretched carbon nanotube array 10, and vice versa. Alength-change rate can be defined as (length after stretching—lengthbefore stretching)/length before stretching×100%. To draw carbonnanotube film 40 from the stretched carbon nanotube array 10, thelength-change rate of the carbon nanotube array 10 can be larger than 0and smaller than or equal to 100%. In one embodiment, the length-changerate of the carbon nanotube array 10 can be about 50%. An area-changerate can be defined as (area of the bottom surface 102 afterstretching—area of the bottom surface 102 before stretching)/area of thebottom surface 102 before stretching×100%. To draw carbon nanotube film40 from the stretched carbon nanotube array 10, the area-change rate ofthe carbon nanotube array 10 can be larger than 0 and smaller than orequal to 300%. In one embodiment, the area-change rate of the carbonnanotube array 10 can be about 125%.

Referring to FIG. 1 and FIG. 10, in block S2, the grooves 12 can beformed before block S3. The grooves 12 are formed on the bottom surface102 of the carbon nanotube array 10 on the elastic substitute substrate30. The grooves 12 can be formed by laser etching the carbon nanotubearray 10. The length of the groove 12 can be a straight line or a curvedline. Referring to FIG. 10, in one embodiment, the plurality of grooves12 can be substantially parallel to each other and along at least twodirections substantially perpendicular to each other. Referring to FIG.11, in another embodiment, one or a plurality of circular shaped grooves12 can be formed on the bottom surface 102 of the carbon nanotube array10 and coaxially arranged with the carbon nanotube array 10. The lengthdirection of at least one groove 12 can be substantially perpendicularto at least one of the stretching directions. The grooves 12 can have adepth that is smaller than the height of the carbon nanotube array 10(i.e., smaller than the length of the carbon nanotubes in the array 10).The length of the grooves 12 can be smaller than or equal to the lengthof the carbon nanotube array 10 along the second direction (y).

Referring to FIG. 12, a laser etching device 80 can be used to emit alaser beam 82 to scan the bottom surface 102 of the carbon nanotubearray 10. The carbon nanotubes absorb the energy of the laser beam 82and heated to react with O₂ gas in air to form CO₂ gas. Therefore, thescanned location of the bottom surface 102 is etched to form grooves 12.The laser etching device 80 can be a pulsed laser generator. The laserbeam 82 can have a power in a range from about 1 W to about 100 W and apower density larger than 0.053×10¹²W/m² on the bottom surface 102. Thedepth of the grooves 12 depends on the power density of the laser beam82 and the scanning speed of the laser beam 82 on the bottom surface102.

The carbon nanotubes in the carbon nanotube array 10 are combined by vander Waals attractive forces. The carbon nanotube array 10 can havestronger van der Waals attractive forces at some places of the carbonnanotube array 10, and weaker van der Waals attractive forces at otherplaces of the carbon nanotube array 10. During the stretching of theelastic substitute substrate 30, cracks can be formed on the surface 102at the places having weaker van der Waals attractive forces, thus makingthe stretched carbon nanotube array less uniform. The grooves 12 formedalong the second direction are artificially weaker places whichdecreases the possibility of crack formation during the stretching alongthe first direction. The carbon nanotubes are not completely removed butshortened by the laser etching, thus the carbon nanotubes are stillcombined by van der Waals attractive forces in the grooves, and thecarbon nanotube film 40 drawn from the etched array 10 is still anintegrated structure. The depth of the grooves 12 can be about 30% toabout 60% of the height of the carbon nanotube array 10. The width ofthe grooves 12 can be in a range from about 10 microns to about 100microns. In one embodiment, the grooves 12 have a depth in a half of theheight of the carbon nanotube array 10 and a width of about 20 microns.The plurality of the grooves 12 can be uniformly spaced from each other.

Referring to FIG. 13 and FIG. 1, the carbon nanotube film 40 is drawnfrom the stretched carbon nanotube array 10 that was transferred to theelastic substitute substrate 30, not from the carbon nanotube array 10located on the growing substrate 20. The carbon nanotube array 10 istransferred to the elastic substitute substrate 30 before the stretchingof the elastic substitute substrate 30 and stretched with and by thestretching of the elastic substitute substrate 30. In one embodiment ofblock S4, the carbon nanotube film 40 can be drawn from the carbonnanotube array 10 upside down on the surface of the elastic substitutesubstrate 30 (i.e., drawn from the bottom surface 102 of the carbonnanotube array 10). The stretched carbon nanotube array 10 has a greaterlength along the first direction caused by the stretching compared withthe original carbon nanotube array 10 grown on the growing substrate 20.Thus, the carbon nanotube film 40 drawn from the stretched carbonnanotube array 10 along the second direction has a greater width. Thesecond direction can be different from the first direction. In oneembodiment, the second direction is substantially perpendicular to thefirst direction. An angle between the second direction and the surface302 of the elastic substitute substrate 30 can also be smaller than 90degrees.

Block S4 can include block S41 and S42.

In block S41, a carbon nanotube segment having a predetermined width isdrawn from the carbon nanotube array 10 on the elastic substitutesubstrate 30. The segment is selected using a drawing tool 50 (e.g.,adhesive tape or other tool allowing multiple carbon nanotubes to begripped and pulled simultaneously);

In block S42, a plurality of carbon nanotube segments joined end to endby van der Waals attractive force is drawn by moving the drawing tool50, thereby forming a continuous carbon nanotube film 40.

In block S41, the carbon nanotube segment includes a single carbonnanotube or a plurality of carbon nanotubes substantially parallel toeach other. The drawing tool 50 such as adhesive tape can be used forselecting and drawing the carbon nanotube segment. The adhesive tape cancontact the carbon nanotubes in the carbon nanotube array to select thecarbon nanotube segment. The drawing tool 50 can select a large width ofcarbon nanotube segments to form the carbon nanotube film. In oneembodiment, an entire length of the carbon nanotube array 10perpendicular to the second direction is selected by the drawing tool50.

In block S42, an angle between a drawing direction of the carbonnanotube segments and the growing direction of the carbon nanotubes inthe carbon nanotube array 10 can be greater than 0 degrees (e.g., 30° to90°).

Blocks A122 and B123 are different from block S4. The purpose of blocksA122 and B123 is to separate the carbon nanotube array 10 as a wholefrom the growing substrate 20. The carbon nanotube array 10 is separatedfrom the growing substrate 20 still in the array shape. The purpose ofblock S4 is to draw out carbon nanotubes one by one or segment bysegment to form a carbon nanotube film or wire from the carbon nanotubearray 10 on the elastic substitute substrate 30.

In the present method for making the carbon nanotube film 40, thegrowing of the carbon nanotube array 10 and the drawing of the carbonnanotube film 40 can be processed on different substrates. The elasticsubstitute substrate 30 can be made of an inexpensive material, and theexpensive growing substrate 20 can be recycled quickly and used againfor growing new carbon nanotube arrays 10, thus speeding up theproduction of the carbon nanotube arrays 10. The carbon nanotube film 40drawn from the stretched carbon nanotube array 10 can have a largertransparency than that drawn from the unstretched carbon nanotube array10.

Depending on the embodiment, certain of the blocks of the methodsdescribed may be removed, others may be added, and the sequence ofblocks may be altered. It is also to be understood that the descriptionand the claims drawn to a method may include some indication inreference to certain blocks. However, the indication used is only to beviewed for identification purposes and not as a suggestion as to anorder for the blocks.

The embodiments shown and described above are only examples. Even thoughnumerous characteristics and advantages of the present technology havebeen set forth in the foregoing description, together with details ofthe structure and function of the present disclosure, the disclosure isillustrative only, and changes may be made in the detail, especially inmatters of shape, size and arrangement of the parts within theprinciples of the present disclosure up to, and including the fullextent established by the broad general meaning of the terms used in theclaims. It will therefore be appreciated that the embodiments describedabove may be modified within the scope of the claims.

What is claimed is:
 1. A method for forming a carbon nanotube film, themethod comprising: providing a carbon nanotube array transferred onto asurface of an elastic substitute substrate, the carbon nanotube arraybeing configured for drawing a carbon nanotube film therefrom, thecarbon nanotube film comprising a plurality of carbon nanotubes joinedend to end; stretching the elastic substitute substrate along aplurality of stretching directions thereby forming a stretched carbonnanotube array having increases in lengths along the plurality ofstretching directions; and drawing the carbon nanotube film from thestretched carbon nanotube array.
 2. The method of claim 1, wherein thecarbon nanotube array is transferred to the elastic substitute substrateby: providing a growing substrate having the carbon nanotube array grownthereon, the carbon nanotube array having a bottom surface adjacent tothe growing substrate and a top surface away from the growing substrate;and transferring the carbon nanotube array from the growing substrate tothe elastic substitute substrate, the carbon nanotube array still beingconfigured for drawing the carbon nanotube film from the elasticsubstitute substrate.
 3. The method of claim 2, wherein the transferringthe carbon nanotube array from the growing substrate to the elasticsubstitute substrate comprises: contacting the surface of the elasticsubstitute substrate to the top surface of the carbon nanotube array;and separating the elastic substitute substrate from the growingsubstrate, thereby separating the bottom surface of the carbon nanotubearray from the growing substrate.
 4. The method of claim 3, wherein thesurface of the elastic substitute substrate and the top surface of thecarbon nanotube array are combined only by van der Waals attractiveforces.
 5. The method of claim 3, wherein the surface of the substitutesubstrate has a plurality of microstructures located thereon.
 6. Themethod of claim 3, wherein the substitute substrate is spaced from thegrowing substrate by a spacing element, the spacing element has a heightbetween the substitute substrate and the growing substrate less than orequal to the height of the carbon nanotube array.
 7. The method of claim2, wherein the transferring the carbon nanotube array from the growingsubstrate to the elastic substitute substrate comprises: placing theelastic substitute substrate on the top surface of the carbon nanotubearray and sandwiching liquid medium between the elastic substitutesubstrate and the carbon nanotube array; solidifying the liquid mediumbetween the elastic substitute substrate and the carbon nanotube arrayinto solid medium; separating the elastic substitute substrate from thegrowing substrate, thereby separating the bottom surface of the carbonnanotube array from the growing substrate; and removing the solid mediumbetween the elastic substitute substrate and the carbon nanotube array.8. The method of claim 7, wherein the sandwiching the liquid mediumbetween the elastic substitute substrate and the carbon nanotube arraycomprises: forming the liquid medium on the top surface of the carbonnanotube array; and contacting the surface of the elastic substitutesubstrate and the liquid medium on the top surface with each other. 9.The method of claim 8, wherein the solidifying the liquid medium betweenthe substitute substrate and the carbon nanotube array comprisescontacting the substitute substrate having a temperature below afreezing point with the liquid medium on the top surface of the carbonnanotube array.
 10. The method of claim 7, wherein the sandwiching theliquid medium between the substitute substrate and the carbon nanotubearray comprises: forming the liquid medium on the surface of thesubstitute substrate; and contacting the top surface of the carbonnanotube array and the liquid medium on the surface of the substitutesubstrate with each other.
 11. The method of claim 7, wherein the liquidmedium is in a shape of a plurality of droplets, mist, or film, and adiameter of the droplet and a thickness of the film is in a range fromabout 10 nanometers to about 300 microns.
 12. The method of claim 7,wherein the solidifying the liquid medium between the substitutesubstrate and the carbon nanotube array comprises placing a laminationof the growing substrate, the carbon nanotube array, the liquid medium,and the substitute substrate into a freezer, the freezer having aninternal temperature below a freezing point of the liquid medium. 13.The method of claim 1, wherein an area-change rate of the carbonnanotube array is larger than 0 and smaller than or equal to 300%. 14.The method of claim 1 further comprising forming at least one groove onthe carbon nanotube array, a length direction of the at least one grooveis substantially perpendicular to at least one of the pluralitystretching directions.
 15. The method of claim 14, wherein the at leastone groove is formed by laser etching.
 16. The method of claim 14,wherein the at least one groove is a circular groove coaxially arrangedwith the carbon nanotube array.
 17. The method of claim 14, wherein theat least one groove comprises a plurality of grooves having lengthdirections along two directions substantially perpendicular to eachother.
 18. The method of claim 14, wherein a depth of the at least onegroove is smaller than a height of the carbon nanotube array.
 19. Themethod of claim 14, wherein a depth of the at least one groove is about30% to about 60% of a height of the carbon nanotube array.
 20. Themethod of claim 14, wherein a width of the at least one groove is about10 microns to about 100 microns.